Pressure sensitive recording sheets of the kind which utilize a reaction between an electron donating color former and an electron accepting color developer are widely known as no carbon required paper or pressure sensitive manifold paper, and specific examples thereof are described in U.S. Pat. Nos. 2,711,375, 2,712,507, 2,730,456, 2,730,457 and 3,617,334, Japanese Patent Publication Nos. 18317/63, 1178/72 and 20972/72, and so on.
Typical representative forms which have so far been employed in preparing and using pressure sensitive manifold paper are as follows. A suitable electron donating color former is dissolved in an appropriate solvent, microencapsulated and coated on a sheet to prepare the "upper sheet". Separately, a suitable electron accepting color developer is coated on a sheet to prepare the "lower sheet". Further, microcapsules of the electron donating color former are coated on one side of a sheet and the electron accepting color developer is coated on the other side of the sheet to prepare the "intermediate sheet". The upper sheet is brought into face-to-face contact with the lower sheet, and when one wants to obtain a number of copies one or more of the intermediate sheets are sandwiched between the upper sheet and the lower sheet. When pressure is applied to a set of the thus superposed sheets using a typewriter or writing means, the set functions as a pressure sensitive manifold paper to produce colored images. The colored images are produced through a process wherein microcapsules present in the pressed areas are destroyed and, thereby, the color formers enclosed on the inside of the microcapsules are transferred into the color developer layer together with an oil to undergo a reaction therein:
The pressure sensitive manifold papers of such a kind are widely used as various office forms, e.g., chits, computing machine paper and so on, and are of very useful.
The most important requirements for a pressure sensitive recording sheet are to have an excellent capacity for coloring by pressure applied thereto using a typewriter or a writing means, and to have little stain at the time of handling thereof, which includes the producing step, the processing step and so on, that is to say, to have little process fog.
Hitherto, in order to decrease fog arising from processing pressures, microcapsule protecting agents have been prevailingly employed. For example, cellulose fine powder (U.S. Pat. No. 2,711,375), starch granules (British Pat. No. 1,232,347, and Japanese Patent Publication Nos. 1178/72 and 33204/73), glass beads (U.S. Pat. No. 2,655,453), heat expansile high polymer particles (Japanese Patent Application (OPI) No. 32013/73 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application")) and so on are known to be usable as the above described agent. However, the use of such microcapsule protecting agents, though enabling reduction of process fog, causes lowering of color density of the developed image upon copying. Accordingly, a measure of increasing the coverage of the microcapsules or the content of the color former has been employed industrially to make up for the lowering of the color density of the developed image. However, these measures suffer from disadvantages of entailing high cost and so on.
Further, a method of previously coating a microcapsule protecting agent of a water-soluble high polymer, such as starch or the like, and coating thereon microcapsules (Japanese Patent Application (OPI) Nos. 111288/80, 150210/79 and 87908/73, and Japanese Patent Publication Nos. 7398/80 and 41365/72) was proposed. However, the effects of such methods upon improvement in the color density are also less than anticipated in the present situation.